This invention relates to the use of block copolymers to modify properties such as fracture toughness of thermoset resin compositions, particularly multifunctional highly cross-linked resin compositions. The invention is of particular utility in epoxy resin compositions. The invention also relates to the use of such thermoset resin compositions in fibre-reinforced composite materials and to structural parts made from such composite materials.
Thermoset materials such as cured epoxy resins have been known for their thermal and chemical resistance. They also display good mechanical properties but they frequently lack toughness and tend to be very brittle. This is especially true as their crosslink density increases or the monomer functionality increases above two. Attempts have been made to strengthen or toughen epoxy resins and other thermoset materials such as bismaleimide resins, benzoxazine resins, cyanate ester resins, epoxy vinyl ester resins and unsaturated polyester resins, and also blends of these, by incorporating therein a variety of elastomeric materials.
A disadvantage of prior art block copolymer toughening agents can be a reduction in the glass transition temperature of the resin composition, which can lead to a decrease in the service temperature of the modified and cured thermoset resin. The service temperature of material is directly correlated to the glass transition temperature of the material. It is described as the highest temperature at which a material can be used without any failure over an extended time period time (typically at least 5000 hours). The “failure” of the material is typically understood as the situation where a property value drops to 50% of its initial value (ASTM-D794). High service temperatures (for instance, around 150 to 160° C.) are required for use in aerospace or high-performance automotive applications, in which a component is required to retain shape and performance at elevated temperatures.
A further disadvantage prior art block copolymer toughening agents can be a reduction in the modulus of the resin.
For certain applications, the toughened resin must also exhibit good solvent resistance.